Aromatic polycarbonates, especially those formed from bisphenol-A and carbonyl chloride, and polyesters, such as poly(ethylene terephthalate), are extensively used in glazing applications. That is, sheets made from these resins are used as windows, films, sky-lights, etc. However, in long-term exposure to ultraviolet (UV) light, the sheets become hazy. The haze is a micro cracking of the surface. This crazing results in a loss of mechanical properties and light transmission. A solution to this problem has been to apply coatings to the sheets. The coating is helpful to some extent since it is able to protect the sheet against haze for a longer period of time than an uncoated sheet. However, crazing is still a problem. Also, coating is expensive since special equipment is needed, and contained in the casting polymer are relatively large quantities of expensive UV absorbers. Further, solvent contained in the coating must be evaporated.
Polyarylates are polyesters derived from a dihydric phenol, particularly bisphenol-A and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. The polyarylates are high temperature, high performance thermoplastic polymers with a good combination of mechanical properties and are used to mold a variety of articles.
It is known that upon exposure to ultraviolet light polyarylates undergo the Photo-Fries rearrangement. Korshak et al in Synthesis and Properties of Self-Protecting Polyarylates, Journal of Polymer Science, Part A-1, Vol. 7, pages 157 to 172 (1969) describe a mechanism to explain what happens when polyarylates are exposed to UV light. The authors propose that polyarylates undergo the Photo-Fries rearrangement resulting in carbonyl and hydroxyl groups ortho to one another, structurally similar to o-hydroxybenzophenones which are known light absorbers. Thus, the polyarylates have increased photostability in the bulk of the material.
Cohen et al in Transparent Ultraviolet-Barrier Coatings, Journal of Polymer Science, Part A-1, Vol. 9, pages 3263 to 3299 (1971) describe a number of phenyl polyesters, including polyarylates, which were synthesized to furnish molecules whose backbones rearrange under UV light to an o-hydroxybenzophenone structure. This Photo-Fries rearrangement produced ultraviolet opacity in an irradiated film while maintaining visual transparency. Thin coatings of these polyesters completely protected substrates ordinarily sensitive to ultraviolet light. The authors state that spectroscopic analysis of various rearranged films and coatings showed that the o-hydroxybenzophenone polymer formed was concentrated at the irradiated surface of the original polyester coating as a "skin". Such a skin, formed in situ during the irradiation, functioned to protect both the original polyester coating as well as the coated substrate from degredation by ultraviolet irradiation. Furthermore, a significant "healing" mechanism appeared inherent in the coatings, for as the exposed skin ultimately degraded under extended ultraviolet irradiation, more of the underlying polyester layer apparently rearranged to compensate for the loss. Thus, the clear coating functioned both as a protective skin and a rearrangeable reservoir.
U.S. Pat. No. 3,492,261 describes film forming solutions of rearrangeable aromatic polyesters which are the polymeric reaction product of an aromatic dihydric compound and an aromatic dicarboxylic compound. It is stated in this patent that the preferred dihydric compounds are the ester derivatives of 4,4-bis(4'-hydroxyphenyl)pentanoic acid. The solutions of aromatic polyesters can be coated onto a substrate and dried to transparent films, are capable of absorbing ultraviolet light and undergo rearrangement in the presence thereof to form a transparent compound which is stable to and will act as a barrier to ultraviolet light.
U.S. Pat. No. 3,503,779 discloses substrates which are coated with rearrangeable aromatic polyesters which are the polymeric reaction product of an aromatic dihydric compound and an aromatic dicarboxyl compound. The outer exposed surface of the aromatic polyester coatings rearrange under the influence of ultraviolet light to form a new transparent compound which is stable to and will act as a barrier to ultraviolet light.
U.S. Pat. No. 3,444,129 discloses rearrangeable aromatic polyesters which are the polymeric reaction product of an aromatic dihydric compound, and an aromatic dicarboxylic compound. The preferred dihydric compound is stated to be the ester derivatives of 4,4-bis(4'-hydroxyphenyl) pentanoic acid. The rearrangeable aromatic polyesters are useful as ultraviolet light barriers.
U.S. Pat. No. 3,460,961 describes substrates which are protected with a transparent aromatic polyester coating of two contiguous superposed layers. The aromatic polyester compounds are the reaction product of an aromatic dihydric compound and an aromatic dicarboxylic compound. The aromatic polyester coating undergoes rearrangement in the presence of ultraviolet light to form a transparent compound which is stable to and acts as a barrier to ultraviolet light.
However, in all of these references coatings are being utilized which, as discussed above, require expensive solvent coating equipment and require evaporation of a solvent.
U.S. patent application Ser. No. 596,075 filed Apr. 4, 1984 in the names of L. M. Robeson et al, titled "Laminate Compositions With Polyarylate and Poly Aryl Ether", commonly assigned, describes laminate compositions comprising a polyarylate, or blends thereof, derived from a dihydric phenol and an aromatic dicarboxylic acid which is laminated upon the surface of a poly(aryl ether), particularly polysulfone. It is stated therein that excellent protection is afforded the polysulfone against degradation resulting from prolonged UV exposure by laminating the polyarylate upon its surface.